Science and industry have developed a need for the ability to accurately detect and measure very small quantities of chemical or biological material. Where it was once adequate to measure quantities in micrograms, today, many applications require the detection of a single cell, subcellular unit, or other small quantity of material, sometimes on the order of 10−15 grams.
For example, within the food industry, even small quantities of particular biological cells or toxins can be harmful or dangerous to mammals. One such well recognized harmful organism is Escherichia coli or E. coli bacteria. Popular media attention concerning the presence of E. coli in meat products, apple juice and alfalfa sprouts has heightened consumer sensitivities.
Sensors for detecting gasses using micromechanical cantilevers are discussed in A Chemical Sensor Based on a Micromechanical Cantilever Array for the Identification of Gases and Vapors, H. P. Lang (IBM Corporation, Saumerstrasse 4, 8803 Ruschlikon, Switzerland) et al., Applied Physics A 66, No. S, S61-S64 (1998) (hereinafter “Lang”). Lang discusses detecting static beam deflection upon exposure of the beam to gases and vapors and measuring resonance frequency shifts based on changes of beam mass due to absorption. Absorbent beams are exposed to a chemical vapor for a period of time (noted in one instance to be several hours) and measurements are taken before the chemicals have evaporated from the beam. Absorption is recognized as largely a result of Lennard-Jones potential, wherein at close distances, nearby molecules repel and at larger distances, the molecules are attracted to each other. In many cases, absorption of molecules onto a surface can be readily reversed by merely heating the system or exposing the system to a vacuum.
Biochemically induced surface stresses in a cantilever array are discussed in Translating Biomolecular Recognition into Nanomechanics, J. Fritz et al, Science, page 316-318, Vol. 288, Apr. 14, 2000 (hereinafter “Fritz”). Fritz discusses absolute deflection of a beam as it relates to ligand binding in a liquid environment.
Detecting mass differences using static deflection of a beam typically requires a more robust beam. In many cases, this means that the beam is dimensionally rather large or the material of which the beam is fabricated has a relatively high Young's Modulus of elasticity. Large beams or those having high Young's Modulus of elasticity can lack the sensitivity needed to detect small quantities of target substances. In addition, beams that acquire additional mass through the process of absorption often require lengthy exposure time to the target substance to accumulate a detectable amount. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a highly sensitive detection system and method that permits the rapid detection of biological or chemical material.